There is an image processing method in which a plurality of images are added and combined to form one image. For example, Japanese Patent Application Laid-Open No. 2003-46859 discusses a method of a tone conversion for appropriately compressing a gradation number of a combined image in the method in which the same subject is imaged a plurality of times with a different exposure amount by a digital camera, and these images are combined to form a combined image having a wide dynamic range.
There has also been discussed a method in which a different subject is imaged a plurality of times, and the plurality of images are added and combined to express a plurality of subjects in one image. In this case, there are two methods: a method in which each of the images is imaged with an appropriate exposure and added, and a method in which each of the images is imaged with an exposure of “1/(the number of times of imaging)” and added. When the background is dark, the former method is effective to ensure the appropriate brightness of each subject, while the latter method is effective to ensure the appropriate exposure after the combining process, during normal image capturing.
When a plurality of image data pieces in which different subjects are captured is combined, and the image data pieces are obtained by the normal image capturing which is not in the dark background, a contrast of the combined image is often reduced if it is obtained only by the simple addition and combination described above, and the each subject may look like see-through in most cases.